In theory, being able to reserve fast passes via smart phone the day you're in the park is a potentially good idea. However, they would seriously need to upgrade their service hardware/software in order to accommodate the instant onslaught of guests who log on at the exact moment the window to reserve opens up so they can get the exact times they want. If the service only works in the park, you'll still have the crowds of hard-core guests at rope drop ... but instead of rushing to get their passes, they'll be standing in a corner crashing the servers.
If they can pull that off, then they also have to accommodate those who do not have smart phones/internet services. I could see them using a sort of computer kiosk (obviously a dozen of them to handle crowds), which they could install in the area where the current princess meet/greet is (once it is moved into the sacred and desecrated ground where SWSA is).
So, when xpass does roll out, will it completely replace the current FP system of going to the attraction, obtaining a ticket, and returning within your window? What about if you decide not to reserve your FP and you're across the park at BTMRR? Do you need to walk all the way back to the front of the park to use their provided method if you don't have a smart phone? And if you do have a smart phone, does reserving the FP online replace the need for there to be physical FP distribution machines?